Recently, white organic EL devices are being actively developed because they can be used for a mono-color display device, a lighting apparatus such as a back light, and a full-color display with color filters. In the case where white organic EL devices are used for lighting applications, they are required to have a high luminous efficiency, for example, which is equivalent to or more than that of fluorescent lamps.
Many methods of producing white light emission by an organic EL device have been disclosed. Few of the methods produce white light with only one kind of emitting material and a single organic EL device generally uses two or more kinds of emitting materials that emit light simultaneously. In the case of using two kinds of emitting materials, a blue emitting material and a yellow-to-red emitting material, yellow-to-red being the complementary color to blue, are selected. However, the yellow-to-red light emission becomes dominant in many cases, thereby yielding a reddish white color.
JP-A-2003-272857 proposes a white organic EL device in which the tendency for red to be strong in emitted light is negated by using a blue emitting layer as an emitting layer on the anode side, the emission range of which tends to be offset in the type where an emitting layer is divided into two layers, and whose color change is suppressed. The level of the luminous efficiency was, however, not necessarily enough.
Further, JP-A-08-078163 discloses a structure having a carrier-recombination-region-control layer interposed between a hole transporting layer and an electron-transporting layer, in which there is obtained white emission having luminous efficiency that is high to some extent although it is not necessarily at a practically sufficient level. Since, however, the affinity level of the above carrier-recombination-region-control layer has a large value relative to the affinity level of the hole transporting layer, the driving voltage is high, and with the passage of the driving time, it comes to be difficult to inject electrons into the hole transporting layer. As a result, the blue emission intensity is decreased, and the emission is liable to be shifted toward red light.
It is an object of the invention to provide an organic EL device with a high luminous efficiency and small change in chromaticity.